The presence of polarization dependent effects (such as, for example, Polarization Dependent Loss/gain (PDL); Polarization Mode dispersion (PMD); Polarization spectral hole burning; Polarization rotation; nonlinearities that are polarization dependent or that otherwise have a polarization effect; polarization dependent filtering or crosstalk; and Faraday effect) can be a limiting factor in the design of optical transmission systems, particularly those providing long haul transmission at bit-rates of 10 Gb/sec or faster, over optical links (e.g. fibers) of 100 kilometers or more in length. The impact of polarization dependent effects (PDEs) in high bandwidth networks is expected to be particularly severe in systems which incorporate cross connected networks of fibers, in which an optical signal can follow any one of a number of possible routes utilizing different fibers (within the same or different cable), each with individual properties. This is particularly so for communications systems utilizing polarization diversity and polarization multiplexing.
The amount of PDE (PDL and/or PMD) varies from fiber to fiber, being dependent upon the amount of intrinsic birefringence associated with core asymmetry or frozen-in stress; extrinsic birefringence associated, for example, with cable induced stress, fiber bends or twists; and polarization coupling between optical elements within a link. As a result of these factors, PDEs tend to be statistical quantities which vary with both wavelength and time.
Optical communications systems suffer degradation attributable to Polarization Dependent Loss (PDL) generally through transients and noise. Transient changes in the polarization couplings along a fiber route cause a transient change in the received power of a polarized signal, which cause errors in the receiver. Furthermore, Amplified Spontaneous Emission (ASE) noise is generally unpolarized, and so PDL along an optical fiber route will tend to attenuate the polarized signal more than the ASE noise travelling with that signal. This will further impair the optical signal-to-noise ratio.
It is known to use polarization scrambling to mitigate the effects of PDL, as disclosed in U.S. Pat. No. 5,416,626, entitled “Optically Amplified Communications Systems”, which issued on May 16, 1995. It is further known to attempt to minimize the PDL introduced by an optical device by improving the design of that device, or controlling the temperature of that device to a fixed value. However, in order to mitigate the effects of PDL inherent in an optical link, it is necessary to compensate PDL dynamically. For WDM communications systems, this functionality must be implemented across a wavelength range that encompasses the optical signal traffic.
Various systems have been proposed for addressing the requirement for broadband dynamic PDL compensation. A typical example is described in “Demonstration of In-Line Monitoring and Dynamic Broadband Compensation of Polarization Dependent Loss” (L. -S. Yan, Q. Yu, and A. E. Willner, paper We.P.38, ECOC'2001). In this system, broadband PDL compensation is achieved by demultiplexing the WDM optical signal to separate each wavelength channel into a respective parallel optical path. The PDL of each of the separated channels is then independently measured and compensated, in parallel, and the thus “PDL-compensated” channels multiplexed back together.
A limitation of this approach is that WDM systems that achieve high spectral efficiencies (e.g., better than about 0.3 bits per second per Hz) generally suffer significant distortion penalties for each multiplexing and demultiplexing function. In addition, per-channel PDL compensation inherently introduces “deadbands” between channels. Within these deadbands, signals cannot be transmitted and PDL cannot be compensated. This tightly ties the PDL compensation system to the particular wavelength plan of the communications system, which is undesirable.
A known method of broadband PDL compensation that avoids deadbands is to impose a selected PDL across a wavelength band of interest (e.g. 5-6 nm wide). However, PDL can exhibit a strong wavelength dependence. Accordingly, the imposed PDL will normally be selected to compensate an average PDL within the wavelength band. While this approach avoids undesirable deadbands, it can only compensate a portion of the PDL within the wavelength band, leaving at least some PDL un-compensated.
Another method of reducing the accumulation of PDL in a optical fiber link is depolarization of the optical signal traffic. Such methods are taught in U.S. Pat. No. 6,205,262, for example. However, depolarization does not permit any dynamic equalization of PDL across a spectrum of channels.
Although the optical fibers used for long-haul optical transmission are nominally “single mode”, propagation is generally characterized by two orthogonally polarized HE11 modes for which slightly different group velocities exist in the presence of birefringence. Accordingly, for an arbitrary polarization state of an optical signal at the input end of the fibre, the optical signal at the output end of the fibre will consist of both polarization modes separated by a certain amount of group delay, or Polarization Mode Dispersion (PMD), which is usually measured in pico seconds per ✓km. Cross-coupling of energy between the polarization modes, in the presence of this differential group delay (DGD), causes waveform distortion and consequent degradation of optical signal quality. Second order PMD produces further signal distortions in the form of differential chromatic dispersion (that is, “polarization-dependent” chromatic dispersion) between the orthogonal polarization modes.
For a detailed description of PMD, see “Long-Term Measurement of PMD and Polarization Drift in Installed Fibers”, Magnus Karlsson et al., Journal of Lightwave Technology, Vol. 18, No. 7, (July, 2000). The effects of PMD in a high bandwidth optical link is discussed in “Temporal Dynamics of Error-Rate Degradation Induced by Polarization Mode Dispersion Fluctuation of a Field Fiber Link”, Henning Bulow et al., Proceedings of the 23rd European Conference on Optical Communications, IOOC-ECOC '97, Edinburgh, UK, Sep. 22-25, 1997. Various methods are known for measuring PMD in an optical transmission system, such as, for example, as disclosed in U.S. Pat. No. 5,949,560 (Roberts et al.); “Automated Measurement of Polarization Mode Dispersion Using Jones Matrix Eigenanalysis”, Heffner, IEEE Photonics Technology Letters, Vol. 4, No. 9, September 1992, pp.1066-1069; and “Measurement of High-Order Polarization Mode Dispersion”, Li et al., IEEE Photonics Technology Letters, Vol. 12, No. 7, July 2000, pp.861-863.
Optical transmission systems using data transmission rates of up to about 10 Gb/s are normally able to tolerate polarization mode dispersion (PMD) on the order of 0.2 pico seconds per ✓km. Future optical transmission systems are expected to achieve data transmission rates of 40 Gb/s (or more), and thus are more likely to be limited by the effects of polarization mode dispersion.
In the prior art, there are three general categories of techniques used for PMD compensation, namely: all-optical, all electrical and hybrid. Of these, all-optical PMD compensation, in which the optical signals remain in the optical domain, is the dominant technique used for PMD compensation in high-speed optical communications.
All-optical PMD compensation methods typically involve the use of a controllable birefringence to impose a differential phase delay to each of the orthogonal HE11 modes. The imposed phase delay is selected to optimize system performance by offsetting at least some of the PMD impressed on the optical signal up-stream of the compensator. These compensation methods typically suffer disadvantages in that the phase delay of controllable birefringent materials typically varies approximately linearly with polarization angle and wavelength. However, PMD is typically non-linear across the range of wavelengths used in WDM optical transmission systems. It is therefore difficult to obtain the desired differential phase delay across the wavelength band of interest. All-optical PMD compensator devices intended to address this problem tend to be highly complex, expensive to manufacture, and suffer from high optical losses.
First order PMD can also be avoided by launching an optical signal on one principle axis of the optical link. This technique is described in U.S. Pat. No. 5,311,346, entitled “Fiber-optic Transmission Polarization-Dependent Distortion Compensation”, which issued on May 10, 1994. In this patent, an optical polarization rotator placed at the transmitter end of an optical link is used to rotate the polarization state of the optical signal to minimize signal distortion measured at the receiver end of the link. This technique is capable of avoiding first order PMD, provided that the PDL is small. However, it cannot mitigate the effects of second or higher order PMD, nor can it contend with the combined effects of both PMD and PDL, when both of these are present in significant amounts.
All-electrical PMD-compensation systems are normally limited to linear electrical filtering in a coherent receiver. Typical examples of such systems are described in “Electronic Equalization Of Fiber PMD-Induced Distortion At 10 Gbit/S” by H. Bulow et al, Optical Fiber Communication (OFC'98), pages 151-152, (1998), and “Equalization in Coherent Lightwave Systems Using a Fractionally Spaced Equalizer” by J. Winters, Lightwave Tech., Vol. 8, No. 01, Oct. 1990, pp 1487-1491. These systems suffer the disadvantage that they are limited to coherent receivers. However, because of their high cost, very few coherent receivers have been installed, and the cost of replacing installed receivers with the high-performance coherent receivers is prohibitive.
The majority of receivers installed in modern optical communications networks are of the direct detection type. Due to the well known squaring effect in these receivers, electrical processing of the output signal is capable of compensating only a very limited amount of PMD. See, for example, “Performance of Smart Lightwave Receivers with Linear Equalization” Cartledge et al, Journal of Lightwave Tech, Vol. 10, No. 8, August 1992, pp. 1105-1109; and “Electrical Signal Processing Techniques in Long-Haul Fiber-Optic Systems” Winters et al, IEEE Trans. Comms, Vol. 38, No. 9, September 1990, pp. 1439-1453}.
Hybrid PMD compensation is a technique that uses both optical and electrical methods to restore the distortion due to PMD. In the article entitled “Polarization Mode Dispersion Compensation by Phase Diversity Detection” by B. W. Hakki, IEEE Photonics Technology Letters, Vol. 9, No. 1, pages 121-123, January 1997, a hybrid PMD compensation technique is disclosed wherein a polarization controller (PC) and a polarization beam splitter (PBS) are used to transform the states of polarization, and split the polarization components. At each output of the PBS, a high-speed photo-detector converts the optical signal to electrical signal. An electrical delay line is used to adjust the phase delay between the two electrical signals.
A cost-effective technique for mitigating effects of polarization on high bandwidth optical signals remains highly desirable.